Microprocessor-accessible memory devices have traditionally been classified as either non-volatile or volatile memory devices. Non-volatile memory devices are capable of retaining stored information even when power to the memory device is turned off. However, non-volatile memory devices occupy a large amount of space and consume large quantities of power, making these devices unsuitable for use in portable devices or as substitutes for frequently-accessed volatile memory devices. On the other hand, volatile memory devices tend to provide greater storage capability and programming options than non-volatile memory devices. Volatile memory devices also generally consume less power than non-volatile devices. However, volatile memory devices require a continuous power supply in order to retain stored memory content.
Commercially viable memory devices that are both randomly accessed and semi-volatile or non-volatile are desired. Various implementations of such semi-volatile and nonvolatile random access memory devices are being developed. These devices store data in a plurality of memory cells by structurally or chemically changing the resistance across the memory cells in response to predetermined voltages applied to the memory cells. Examples of variable resistance memory devices being investigated include memories using variable resistance polymers, perovskite, doped amorphous silicon, phase-changing glasses, or doped chalcogenide glass, among others.
In a variable resistance memory cell, a first value may be written to the variable resistance memory cell by applying a voltage having a predetermined level. The applied voltage changes the electrical resistance across the memory cell. A second value, or the default value, may be written or restored in the memory cell by applying a second voltage to the memory cell, thereby changing the resistance across the memory cell to the original resistance level. The second voltage is typically a negative voltage in comparison to the first voltage and may or may not have the same magnitude as the first voltage. Each resistance state is stable, so that the memory cells are capable of retaining their stored values without being frequently refreshed. The variable resistance materials can thus be “programmed” to any of the stable resistance values.
The content of a variable resistance memory cell is read or “accessed” by applying a read voltage to determine the resistance level across the cell. The magnitude of the read voltage is lower than the magnitude of the voltage required to change the resistance of the variable resistance memory cell. In a binary variable resistance memory cell, upon determining the resistance level of the variable resistance memory cell, the detected resistance level is compared with a reference resistance level. Generally, if the detected resistance level is greater than the reference level, the memory cell is determined to be in the “off” state. On the other hand, if the detected resistance level is less than the reference level, the memory cell is determined to be in the “on” state.
FIG. 1 shows a basic composition of a variable resistance memory cell 10 constructed over a substrate 12, having a variable resistance material 16 formed between two electrodes 14, 18. One type of variable resistance material may be amorphous silicon doped with V, Co, Ni, Pd, Fe and Mn as disclosed in U.S. Pat. No. 5,541,869 to Rose et al. Another type of variable resistance material may include perovskite materials such as Pr(1-x)CaxMnO3 (PCMO), La(1-x)CaxMnO3 (LCMO), LaSrMnO3 (LSMO), GdBaCoxOy (GBCO) as disclosed in U.S. Pat. No. 6,473,332 to Ignatiev et al. Still another type of variable resistance material may be a doped chalcogenide glass of the formula AxBy, where “B” is selected from among S, Se and Te and mixtures thereof, and where “A” includes at least one element from Group IIIA (B, Al, Ga, In, Tl), Group IVA (C, Si, Ge, Sn, Pb), Group VA (N, P, As, Sb, Bi), or Group VIIA (F, Cl, Br, I, At) of the periodic table, and with the dopant being selected from among the noble metals and transition metals, including Ag, Au, Pt, Cu, Cd, Ir, Ru, Co, Cr, Mn or Ni, as disclosed in U.S. Published Application Nos. 2003/0045054 and 2003/0047765 to Campbell et al. and Campbell, respectively. Yet another type of variable resistance material includes a carbon-polymer film comprising carbon black particulates or graphite, for example, mixed into a plastic polymer, such as that disclosed in U.S. Pat. No. 6,072,716 to Jacobson et al. The material used to form the electrodes 14, 18 can be selected from a variety of conductive materials, such as tungsten, nickel, tantalum, titanium, titanium nitride, aluminum, platinum, or silver, among others.
In FIG. 2, a typical prior art variable resistance memory cell 100 is shown to include an access device 102, a variable resistance memory element 104, and a cell plate 110. The access device 102 is a transistor having a gate 102a coupled to a word line 106 and one terminal (source) 102b coupled to a bit line 108. The other terminal (drain) 102c of the access device 102 is coupled to one end of the variable resistance memory element 104, while the other end of the variable resistance memory element 104 is coupled to the cell plate 110. The cell plate 110 may span and be coupled to several other variable resistance memory cells, and may form the anode of all the memory elements 104 in an array of variable resistance memory cells. The cell plate 110 is also coupled to a potential source 112.
A representative diagram of the physical structure of the prior art memory cell 100 is shown in FIG. 3. In particular, a p-doped substrate 126 includes two n-wells 120, 122. Access device 102 is formed on the surface of the substrate 126 between the two n-wells 120, 122, so that the two n-wells 120, 122 serve as the source 102b and drain 102c, respectively, of the access device 102. Word line 106 is formed as a conductive strip extending into the page across the top of access device 102. Bit line 108 is connected directly to the n-well 120 forming the source 102b of the access device 102. Variable resistance memory element 104 is formed on the substrate with its cathode 114 in contact with n-well 122 and the cell plate 110 (only a portion of which is shown) as its anode. The cell plate 110 of memory element 104 is connected to a potential source 112.
In the conventional operating scheme for the cell 100, when the memory element 104 is idle, the voltage across the anode 110 and the cathode 114 is below a threshold voltage VG. The value of the threshold voltage VG is a function of the specific variable resistance material used in the memory element 104. In order to perform any access operations including programming the variable resistance memory element 104 to the low resistance state, erasing a programmed variable resistance memory element 104 by returning the variable resistance memory element 104 to the high resistance state, or reading the value stored in memory element 104, the threshold voltage VG must be applied to the word line 106. The voltage VG on the word line 106 activates the gate 102a of the access device 102 so that an n-channel is formed in the substrate 126 under the gate structure of the access device 102 and across the gap between the two n-wells 120, 122 thus activating the access device 102. Upon activating the access device 102, the memory element 104 can be programmed to the low resistance state by applying a write (positive) voltage having at least the magnitude of a threshold voltage VTW across the memory element 104.
In conventional operating schemes, application of the write voltage may be achieved by raising the potential at the cell plate 110 (anode) relative to the access device drain 102b by applying or raising the voltage at the potential source 112, lowering the potential of the bit line 108, or a combination of both. To erase a programmed memory element 104, a negative voltage having a magnitude of at least a threshold erase voltage is applied between the anode and the cathode of the memory element 104, such that the potential at the cell plate 110 is lower than the potential of the bit line 108.
Variable resistance memory cells are arranged as an array of memory cells and are written, erased, and read using a controller. FIG. 4 illustrates a prior art memory device 200 comprising an array of memory cells 100a-100f arranged in rows and columns. The memory cells 100a-100f along any given bit line 108a, 108b do not share a common word line 106a-106c. Conversely, the memory cells 100a-100f along an), given word line 106a-106c do not share a common bit line 108a-108b. In this manner, each memory cell is uniquely identified by the combined selection of the word line to which the gate of the memory cell access device is connected, and the bit line to which the source of the memory cell access device is connected.
Each word line 106a-106c is connected to a word line driver 202a-202c via a respective transistor 204a-204c for selecting the respective word line for an access operation. The gates of the transistors 204a-204c are used to selectively couple or decouple the word lines 106a-106c to or from the word line drivers 202a-202c. Similarly, each bit line 108a, 108b is coupled to a driver 206a, 206b via selector gates 208a, 208b. The current and/or resistance of a selected memory cell 100a-100f is measured by sensor amplifiers 210a, 210b connected respectively to the bit lines 108a, 108b. 
For simplicity, FIG. 4 illustrates a memory array having only two rows of memory cells 100 on two bit lines 108a-108b and three columns of memory cells 100 on three word lines 106a-106c. However, it should be understood that in practical applications, memory devices would have significantly more cells in an array. For example, an actual memory device may include several million memory cells 100 arranged in a number of subarrays.
While the overall operating scheme of the memory device 200 may be similar regardless of the type of variable resistance material used in the memory elements, much research has focused on memory devices using memory elements having doped chalcogenide materials as the variable resistance material. More specifically, memory cells having a variable resistance material formed of germanium-selenide glass having a stoichiometry of GexSe(100-x), with x ranging from about 20 to about 43, which are doped with metal ions, have been shown to be particularly promising for providing a viable commercial alternative to traditional random-access memory devices.
Generally, a metal ion doped chalcogenide variable resistance memory cell having such stoichiometry has an initial “off” state resistance of over 100 k (for example, 1 M). To perform a write operation on a chalcogenide memory cell in its normal high resistive state, a voltage having at least a threshold potential is applied to the electrode serving as the anode, with the cathode held at the reference potential or ground. Upon applying the threshold level or write voltage, the resistance across the memory cell changes to a level dramatically reduced from the resistance in its normal state. The new resistance of the memory cell is less than 100 k (for example, 20 k). The cell is considered to be in the “on” state while in the low-resistive state.
The variable resistance memory cell retains this new lower level of resistivity until the resistivity is changed by another qualifying voltage level applied to one of the electrodes of the cell. For example, the memory cell is returned to the high resistance state by applying an erase voltage thereto in the negative direction of the voltage applied in the write operation (to achieve the lower resistance state). The erase voltage may or may not be the same magnitude as the write voltage, but is at least of the same order of magnitude.
Such chalcogenide variable resistance memory cells can retain a low-resistance state for several hours, days, or even weeks and are relatively non-volatile compared with typical random-access memory devices. However, while metal ion doped chalcogenide variable resistance memory cells in the high resistance state are completely non-volatile, variable resistance memory cells written to the low resistive state may gradually lose their conductivity across the chalcogenide glass layer and drift towards the high resistive state after an extended period of time. In particular, it has been found that metal ion doped chalcogenide variable resistance memory devices which are written using write voltages with pulse widths of less than 100 ns have a tendency to gradually lose their low resistance characteristic in as little as a week. Accordingly, such variable resistance memory devices may require some intermittent refreshing to maintain optimal operation of the devices.
In addition to intermittent refresh operations, metal ion doped chalcogenide variable resistance memory cells may require an occasional reset operation to reset the bistable resistance levels. Over time, the resistance levels resulting from application of various threshold voltages tend to drift. The drifting voltage/resistance (V/R) relationship is further explained below in the context of write and erase operations via measured voltage/resistance curves.
A standard voltage/resistance curve for a write operation performed on a properly functioning metal ion doped chalcogenide variable resistance memory cell is illustrated in FIG. 6A. A voltage/resistance curve, such as that shown in FIG. 6A, is derived by measuring the resistance across the chalcogenide variable resistance memory cell as a function of voltage for a given current. The initial or normal resistance level of a chalcogenide variable resistance memory cell is shown as ROFF, which is above a minimum threshold level REMin in which the chalcogenide variable resistance memory cell is stable in a high resistance state. When the chalcogenide variable resistance memory cell is in the high resistance state and VTW is applied to the cell, the resistance drops to the level indicated by RON, which is below a maximum threshold level RWMax in which the chalcogenide variable resistance memory cell is stable in a low resistance state.
FIG. 5B shows the same programming circuit 300 illustrated in FIG. 5A, except that an erase voltage VTE is applied to the bottom electrode 114 to illustrate an erase operation. By way of example, VTE has a voltage level of −0.75 V and a pulse width of about 8 ns. Upon the application of VTE to the bottom electrode 114 of the chalcogenide variable resistance memory cell 310, the chalcogenide variable resistance memory cell 310 returns to its high resistance (“off”) state, thus erasing the binary value of “1” previously written in the cell, so that the value of “0” is again stored in the chalcogenide variable resistance memory cell 310.
FIG. 6B shows a typical voltage-resistance curve for a metal ion doped chalcogenide variable resistance memory cell during an erase operation. As in FIG. 6A, RON indicates the resistance level of the memory cell in the low resistance (“on”) state, and RWMax represents the maximum resistance value at which the memory cell is stable in the low resistance state, while ROFF indicates a resistance level of the memory cell in the high resistance (“off”) state, and REMin demonstrates the minimum resistance value at which the memory cell is stable in the high resistance state. When the metal ion doped chalcogenide variable resistance memory cell is in the low resistance state and VTE is subsequently applied to the cell, the resistance in the chalcogenide variable resistance memory cell increases to the level indicated by ROFF. It is noted that the write voltage VTW is not necessarily of the same magnitude as the erase voltage VTE.
However, as mentioned above, the resistance profiles of metal ion doped chalcogenide variable resistance memory cells have a tendency to shift after a number of read or write operations have been applied to the cell. Specifically, the cell may eventually be written into an “on” state in which the resistance in that state is unacceptably high or unacceptably low, or an erase operation may place the cell in an “off” state in which the resistance in that state is unacceptably low or unacceptably high. This can happen in as few as about 400 write and erase cycles. Typical life expectancies for random access memory devices are on the order of 1014 write/erase cycles. Thus, the resistance drift should be corrected for longevity of operation of the memory cell.
The phenomenon of resistance drift is demonstrated in FIG. 7, which depicts the case when the chalcogenide variable resistance memory cell drifts towards a low resistance “off” state RDE, meaning that after repeated cycles over time, the “off” state resistance achieved upon application of the fixed erase voltage VTE falls below the level ROFF shown in FIG. 6B. Similarly, the memory cell exhibits an unusually low resistance “on” state RON. The resistance RON becomes progressively more variable and drifts increasingly lower upon the performance of repeated erase cycles until application of the threshold erase voltage VTE is consistently insufficient to bring the memory cell to the minimum stable high resistance level REMin, as illustrated in FIG. 6. Once this condition is reached, subsequent erase operations will fail to erase the stored value in the chalcogenide variable resistance memory cell, causing a breakdown in the function of the chalcogenide variable resistance memory device. Additionally, continued write cycles applied to these already low resistance state memory cells result in pushing the memory cells into an even lower resistance state.
A solution to the voltage/resistance curve shift problem described above and illustrated in FIG. 7 is to periodically reset the memory cells to an original high resistance level. An applied reset or “hard” erase pulse serves to reestablish the original resistance profile of the memory cell in the high resistance state. The “hard” erase pulse may be applied by increasing the voltage level and/or the pulse width relative to erase voltage VTE applied in a normal erase operation. A “normal” erase pulse is illustrated in FIG. 8A having, for example, a voltage level of −0.8 V and a duration of 8 ns. A first type of “hard” erase pulse is shown in FIG. 8B, in which the applied pulse is the same duration as the normal erase pulse, but has a negative voltage level of a magnitude greater than the −0.8 V of the normal erase pulse. An alternative “hard” erase pulse is shown in FIG. 8C, in which the “hard” erase pulse has the sane magnitude as the normal erase pulse, but has a longer pulse width. In a further alternative, the “hard” erase pulse may have both a greater magnitude and a longer duration than the normal erase pulse. The amount by which the voltage level or the duration of the “hard” erase pulse exceeds that of the normal erase pulse may vary depending on the amount of drift, or the amount by which RDE falls below REMin.
A significant challenge exists in determining an appropriate magnitude of an applied “hard” erase voltage pulse, as illustrated in FIGS. 9A, 9B and 9C. A proposed metal ion doped chalcogenide variable resistance memory cell 500 is depicted in FIG. 9B, comprising a p-doped substrate 526 and two n-wells 520, 522. An access device 502 is depicted as a transistor, and is shown in both FIGS. 9A and 9B. The access device 502 is activated by an “on” word line 506 (i.e., a word line having a voltage sufficient to activate access device 502), which is connected to the gate of the transistor. The active access device 502 allows current to flow between the bit line 508 (connected to the source of the transistor) and the cell plate 510 (connected through the variable resistance memory element 504 to the drain of the transistor). In other words, when the access device 502 is activated, current must flow through two series resistances (refer to FIG. 9C). One resistance, the cell resistance, Rcell, is highly variable, due to tie drifting RON and RDE levels, as described in detail above. Typically, Rcell can range anywhere from 5 k● to 46 k●. The other resistance that must be accounted for is the channel resistance, Rchannel, which is generally about 30 k●. With both resistances Rcell, Rchannel in series, a voltage divider equation must be solved in order to determine the voltage across the chalcogenide variable resistance memory cell 504.
                              V          cell                =                                            (                                                V                  BL                                -                                  V                  cpin                                            )                        *                          R              cell                                                          R              cell                        +                          R              channel                                                          Eq        .                                  ⁢        1            Using Equation 1, and setting (for purposes of example only) VBL to equal 2.2 V and Vcpin to equal 0.6 V, the value of Vcell will range from 0.23 V to 0.97 V as a result of the variable resistance of Rcell. In other words, if the resistance-voltage curve for the chalcogenide variable resistance memory cell has drifted too low, as shown in FIG. 7, then a normal “hard” erase voltage applied to VBL and Vcpin may not create a sufficient differential to actually reset the chalcogenide variable resistance memory cell.
From the discussion above, it should be appreciated that an improved method for effectuating a “hard” erase of a chalcogenide variable resistance memory cell is both needed and desired.